The present invention relates to a semiconductor integrated circuit and more particularly, to a semiconductor integrated circuit which is preferably applied to a television sound signal processing circuit especially with an integrated filter circuit therein.
FIG. 4 is a characteristic diagram showing frequency allocation in a television signal transmitted from a broadcasting station, and shows, in Japan, for instance, frequency allocation in a signal whose frequency is converted to a prespecified frequency within a low band by a tuner. In Japan, a carrier of a video signal is set to 58.75 MHz, and a carrier of a sound signal is set to 54.25 MHz which is lower by 4.5 MHz as compared to the carrier of the video signal.
In Japan, as shown in FIG. 5, a sound carrier is FM-modulated with a sound signal (FIG. 5A), the FM-modulated signal (FIG. 5B) is superimposed on a composite video signal (FIG. 5C), and the carrier of 58.75 MHz is AM-modulated by the superimposed signal (FIG. 5D). The frequency of the sound carrier is different depending upon the system such as the NTSC system, the PAL system, and the SECAM system, and 5.5 MHz, 6.0 MHz, and 6.5 MHz are used as sound carriers other than 4.5 MHz.
FIG. 6 is a block diagram showing a conventional type of detector for detecting a sound carrier signal of 4.5 MHz, and shows a detector based on a sound detecting system called an inter-carrier system. This detector comprises an AM detector 11, a voltage-controlled oscillator 12, a band-pass filter (BPF) 13 for 4.5 MHz, a limiter (LIM) amplifier 14, and an FM detector 15. Inputted into the AM detector 11 are a sound intermediate frequency (IF) signal superimposed on a video intermediate frequency (IF) signal and an output signal, whose frequency synchronizes to that of the video intermediate frequency signal for frequency conversion, from the voltage-controlled oscillator 12.
The AM detector 11 demodulates the video intermediate frequency signal and the sound intermediate frequency signal, and outputs a video signal and a sound FM signal at 4.5 MHz. As an AM detector 11, there is, for instance, a balanced modulator which outputs a sum and a difference of two input signals. For example, when frequencies of the sound intermediate frequency signal and that of the output signal from the voltage-controlled oscillator 12 are 54.25 MHz and 58.75 MHz respectively, the balanced modulator outputs signals of 4.5 MHz and 113 MHz.
Unwanted signal components excluding the sound FM signal in a low band are removed by the band-pass filter 13 from the output signals of the AM detector 11. The limiter amplifier 14 maintains the amplitude of the FM signals having passed through the band-pass filter at a constant amplitude, and the FM detector 15 FM-demodulates the FM signals and converts them to the original sound signals.
FIG. 7 is a block diagram showing a detector based on a sound detecting system called a split carrier system. This detector comprises a voltage-controlled oscillator 12, a band-pass filter (BPF) 13 for 4.5 MHz, a limiter (LIM) amplifier 14, an FM detector 15, a frequency converter 16, and an automatic gain controller 17. Inputted into the frequency converter 16 are a sound intermediate frequency signal with some video signal components removed by a SAW (surface acoustic wave) filter provided in the previous stage which is not shown and an output signal from the voltage-controlled oscillator 12.
The frequency converter 16 converts the sound intermediate frequency signal into a low sound FM signal with a frequency of 4.5 MHz or so. This converted sound FM signal substantially comprises only a sound carrier signal, and its amplitude is controlled to be constant by the automatic gain controller 17. The sound FM signal whose gain has been controlled is converted into an original sound signal by the FM detector 15 via the band-pass filter 13 and the limiter amplifier 14.
In the inter-carrier system, it is difficult to obtain sound carrier signals with constant amplitude when an amplitude ratio between a video intermediate frequency signal and a sound carrier signal of received signals or loss of a sound carrier in the SAW filter provided in the previous stage of the detector are different from each other. However, sound carrier signals with constant amplitude can be obtained in the split carrier system by providing automatic gain control specific to a sound carrier rather than the signals, and these signals can be sent to the FM detector 15.
Conventionally, the band-pass filter 13 has been an external component to a semiconductor integrated circuit comprising other circuits excluding this band-pass filter 13 in both the inter-carrier system and the split carrier system. However, when a detector is to be downsized by reducing the number of discrete components and also an integrated filter is to be provided in the semiconductor integrated circuit to make easier assembly of a circuit board for a television receiver, the following inconveniences may occur. Namely, the filter comprises a set of a passive element such as a resistor and a capacitor and an active element, but those elements generate noise. Therefore, when the filter is built in a semiconductor integrated circuit, the S/N (a signal-to-noise ratio) is degraded.
It is an object of the present invention to obtain, for the purpose of solving the problems described above, a semiconductor integrated circuit with an optimally integrated filter therein in both the split carrier system and the inter-carrier system without degradation of a signal-to-noise ratio.
In the present invention, a semiconductor integrated circuit comprises, on an identical semiconductor substrate, a band-pass filter for extracting only the signals within a substantially specified frequency band from the demodulated input signals, an automatic gain controller for detecting an amplitude of the signals extracted by the band-pass filter and controlling so as to keep the amplitude thereof constant, an amplitude detector for demodulating the signals having passed through the band-pass filter, and a switching element for switching the signal to be inputted into the automatic gain controller to a signal to be inputted into the band-pass filter or to a signal outputted from the band-pass filter, and the automatic gain controller is controlled by the amplitude detector according to output from the switching element.